Annulenes structure

The 5,10-epoxy[10]annulene system readily undergoes addition of bromine with formation of an epoxide across the central bond. By dehydrohalogenation the annulene structure can be restored. For example, the dibromo and tetrabromo adducts of 5,10-epoxy[10]annulene can be converted with potassium rm-butoxide to l-bromo-5,10-epoxy[10]annulene (13) and 1,6-dibromo-5,10-epoxy[10]annulene (14), respectively.159... 

Ando and coworkers23 reacted C(,o with bis(2,6-diisopropylphenyl)silylene generated in situ by the photolysis of the corresponding trisilane. The dark brown powder obtained (58%) exhibits 29Si signal at 8 — 72.74 which, together with other experimental evidence, supports the silirane structure (8a) rather than the bridged annulene structure (8b). 

Prior to the work of Cheng et al. Rubin et al. reported a similar [4 + 4] pho-tocyclization reaction [299]. Rubin has shown that a cyclohexadiene derivative underwent a very facile photochemically promoted rearrangement to the stable bridged bisfulleroid 198 (Scheme 76). This process occurs via the initial [4 + 4] photoadduct (not observed), which undergoes a thermally allowed [2 + 2 + 2] cycloreversion to afford a bis-methano[12]annulene structure 198. Alternatively, compound 198 can be obtained by photolysis of the allylic alcohol 197 under reflux and acidic conditions in good yields. The allylic alcohol is obtained by acidic... 

Apparently, the first examples of [16]annulenoid porphyrin derivatives were obtained by Eschenmoser and coworkers during their work on corphin (21, Fig. 15), a peripherally reduced porphyrin derivative [91]. Free base corphin has the appropriate oxidation level to adopt a tetraaza[16]annulene structure, but it actually exists as the cross-conjugated tautomer 21. However, some of the complexes of corphin with divalent metals (such as Zn or Pd) were shown to have the symmetrical structure 22, and their H NMR spectra revealed a modest paratropic ring current, comparable with that of [16]annulene [92-94],... 

In this case the bridged annulene structure causes extreme steric hindrance to peripheral conjugation and the ring is also deformed. Hence there is insufficient delocalisation energy to favour the bridged annulene structure, and this, together with steric factors, leads to (XXXII) being the preferred structure. 

Recently, a hybrid radiaannulene 90 containing electron-donating TTFs and electron-accepting TEE moieties has been prepared by Nielsen and coworkers [86]. This system can be considered as a two-stage Weitz-type redox system, which leads to the aromatic 144t-annulene structure by either oxidation or reduction. Thus, CV analysis clearly showed three reversible oxidation waves corresponding to the formation of three redox states (+1/+2/+4) and showed two reversible reduction waves corresponding to two redox states (—1/—2). From the... 

The different stereochemical features of thermal and photochemical [(,2 + 4] processes are nicely summarized in the reactions of I l6]-annulene, (Equation 6.31). The cyclizations occur at the tennini of the trans,cis,trans-hexatriene units of the [ 16]-annulene structure. 

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